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Choose when to block your phone number

Prevent your cell phone number and information from being collected by blocking your phone number. Allow family and friends to identify your calls by adding a prefix to their numbers in your contacts.

Jason Cipriani/CNET

Your cell phone number is one of the many items of personal information that are bought and sold via the Internet.

The Privacy Rights Clearinghouse fact sheet on protecting your telephone records describes businesses that sell information related to any cell phone number you provide them, including the name and address associated with the number.

Plenty of businesses you call collect your phone number and any other caller-ID information that you provide. You have to imagine that some (most?) of those businesses are selling, trading, or otherwise using the information they collect about their customers.

That's why it makes sense not to share your caller information with every person or business you call. On the other hand, many people automatically reject calls from private numbers.

To ensure the people closest to you can identify your calls, you can override caller ID blocking by entering either *82 or *31 before the number. What's missing is a single setting that allows all the calls you make to your contacts to identify you -- and all other calls to hide your ID.

Four useful prefixes
Every cell phone should come with the option to enable caller ID when you're calling a number that's listed in your contacts. Instead, caller-ID blocking is all or nothing. Even worse, the process for enabling and disabling caller ID depends on your service provider and type of phone.

For example, on an AT&T iPhone, you can block your caller ID in Settings > Phone > Show My Caller ID and then toggle the switch to off.

For other carriers and phone models, disabling caller ID for your outgoing calls may require signing into your account on your service provider's site to change the settings, or calling the company's toll-free support number. (Note that your number isn't blocked when you call 911, toll-free numbers, and some other business numbers.)

You can block all incoming calls except those from the numbers you choose, as I described in last February's "How to screen unwanted calls on iPhones and Android phones."

iPhone contact number with caller-ID-unblock prefix
Edit the phone numbers of family, friends, and associates in your contacts to add the prefix that unblocks caller ID. Screenshot by Dennis O'Reilly/CNET

The simplest approach for selective outbound caller ID is to add one of two unblocking prefixes to each contact's phone number. Which of the two asterisk ("*") prefixes to use depends on your phone and carrier.

At least one telephone star command is so popular it's a verb, as in "I *69ed him to find out which bar he was calling from." You may also know that you can enter *67 to enable block your number for an individual call, *82 to disable caller-ID blocking, and *70 to disable call waiting.

Unfortunately, *82 doesn't work on iPhones using AT&T service (*67 may not work, either). On these phones, the prefix that both disables and enables caller ID blocking is *31, which reverses the setting whether it's set to show or hide your phone number.

The Verizon support site provides instructions for using caller ID on phones on their network. Likewise, the AT&T support site explains how to use the caller ID features on its iPhones and other models. T-Mobile's Blocking Anonymous Callers page refers you to the instructions that came with your phone, or suggests that you call the company's support number.

It's a hassle to enter the unblock prefix for family, friends, and business associates one contact number at a time. But until phone companies offer a one-click, contacts-only, caller-ID option, the manual approach is your best bet.